1. (Field of the Invention)
The present invention generally relates to a steel connecting rod of type having its opposite ends having a bearing built therein and, more particuarly, to the steel connecting rod for a small-size internal combustion engine such as used in a mower.
2. (Description of the Prior Art)
For reduction in weight and manufacturing cost, the connecting rod employed in a small-size internal combustion engine is generally of a structure comprising two connecting rod components, each in the form of an elongated steel plate, that are joined in face-to-face relation together. Different examples of the prior art connecting rods are shown in FIGS. 7A and 7B and FIGS. 8A and 8B, respectively. Each of the steel connecting rod components 51 has its opposite ends formed with respective bearing holes each being in the form of a burring 52, and a bearing (not shown) such as, for example, a shell-type needle roller bearing is press-fitted into each of the bearing holes so as to extend between the steel connecting rod components 51. For increasing the strength of a generally flattened shank 53 against buckling, the steel connecting rod components 51 are formed at side edges with respective reinforcement flanges 55 as shown in FIG. 7A or formed at a longitudinal mid-center area with reinforcement ribs 56 as shown in FIGS. 8A and 8B.
However, when the bearing load is large, the burrings 52 tend to exhibit an insufficient strength. Increase of the plate thickness may result in increase of the strength, but the increase of the plate thickness is often limited because of a shaping work in a limited space and also because of the necessity of reducing the weight. Although it may be contemplated to increase the strength by the use of a heat treatment, the heat treatment may be accompanied by reduction in precision to which the machining is carried out and, therefore, increase in strength by the use of the heat treatment cannot be achieved without sacrificing the precision.